The present invention relates, in general, to the field of microprocessors (MPUs) and microcomputers (MCUs) having integral system control registers. More particularly, the present invention relates to a write protection circuit and method for a control register which allows a write protected register to be written only once within a designated period of time after system reset.
In an MPU/MCU system, certain control registers and bits which determine system configuration would pose serious system integrity problems were they writeable, or under software control, during normal operation. An accidental write to these registers could change the system configuration resulting in a system failure. In the past, such configuration options in MPU/MCU systems were handled with separate mask set variations during fabrication of the devices or were otherwise not available. It would therefore be highly desirable to provide accessibility to such control registers to determine system configuration utilizing a single mask set yet maintaining a high degree of system security and integrity. Moreover, if it were possible to provide writeable control registers for various MPU/MCU system functions which afforded system security and integrity, numerous of such system functions could be placed under software control thus allowing for enhanced device operation.